I'm sure someone from the FBI is saying: Thanks for all the ideas guys!
The problem with the 4d chess approach (leaving it in plain sight) is that the FBI doesn't have a time limit, they can just search every USB they find, which would definitely include anything in plain sight. They might keep looking for a hidden one, but eventually they'd know they have the real thing. Disguising it as a non-USB bit of electronics might work, but I suspect the FBI would search all of that eventually, too. If I'm the FBI and can't find the specific USB on site, then I would take all the electronics I can find back to a lab for further scrutiny, so if the USB is among that, it would be discovered.
Despite being outside the scope of the question, the "not on my property" or destruction answers are probably the best real world answers. You would have to be careful with somewhere else though: you don't want to tip off the FBI that they need a warrant for your friend's house or wherever else.
Places I thought about include:
1. Somewhere in the attic insulation (would be discovered by someone using a metal detector up there)
2. Inside the water heater (where the insulation is) or some other appliance, but I suspect the FBI would search through all the appliances anyways
3. Inside a sink drain or toilet tank, those are commonly searched areas though
4. Taped inside an outlet or light switch, also commonly searched areas